The invention generally relates to the reception of digital signals transmitted on a TV channel.
Many systems use the existing TV emitter network for transmitting digital data. For example, one can cite systems for transmitting messages or systems for triggering an actuator, for example an automatic video tape recorder start at the beginning of a TV programme.
Generally, digital signals are emitted within the free time period existing during the frame retrace period of the TV signal.
It is assumed in the following that the TV receiver has conventionally identified the frame retrace periods and is therefore capable of establishing, at least approximately, windows during which digital signals are liable to appear. More particularly, during each frame retrace period, two windows are formed, the first one during which appear emitted signals constituted by an alternate succession of zeros and ones and the second window during which appear the desired data signals.
In order to detect (or to extract) digital data, it is necessary, at each incoming data clock cycle, to determine whether the level is high or low (1 to 0), that is, to compare the incoming data with one (or two) decision thresholds. On the other hand, this comparison is to be made at an appropriate time during the clock cycle, that is, it is necessary to adjust the clock phase.
A recognition device is very schematically illustrated in FIG. 1: the incoming signals V.sub.IN are supplied to the first input of a comparator 1, the second input of which is connected to a threshold voltage V.sub.T ; the comparator output is connected to a flip-flop 2 actuated by a clock f0 corresponding to the digital data rate.
This analysis is delicate due to the various parasitic pulses added to the video signal. FIG. 2 illustrates an exemplary digital signal comprising, during a window W1, an alternate succession of zeros and ones, then, during a window W2, a digital word, for example 111100111000 . . . As illustrated in FIGS. 2B and 2C, the signal arriving on comparator 1 is not the ideal signal of FIG. 2 but a signal having a pass-band intentionally limited at the emission and impaired by the transmission.
On the one hand, as shown by comparing FIG. 2B with FIG. 2C, the digital data amplitude is liable to vary within an important ratio, usually ranging from 1 to 4 if, as conventional, the gain of the video signal input receiver is controlled on the average amplitude of the video signal during the frame. Thus, this gain will be high in case of a dark image (case of FIG. 2C) and low in case of a light image (case of FIG. 2B).
On the other hand, the phase and amplitude of the signals are impaired by filtering and adjustment inaccuracies at the TV input, as well as by echo receptions and other parasitic pulses.
Lastly, the random noise accompanying any transmitted signal may be significant.
As a result, the threshold voltage V.sub.T with which is compared the input signal V.sub.IN has to be adjusted as a function of the input signal amplitude. It is desirable that the level V.sub.T is adjusted to a value substantially equal to half the peak amplitude of the input signal. Then, fluctuations associated with noise are avoided. On the other hand, it is desirable that detection is made substantially in the middle of the time duration of a 1 or a 0 in order to be in the area where the noise has the lowest influence.
In the prior art, in order to solve this problem, one has resorted to approaches of the analog or digital type.
The conventional analog-type approaches consist in storing, after passage in a low-pass filter, the peak level on a capacitor for subsequently supplying from that point the detection level. It is also possible to detect and store on a capacitor the maximum and minimum data levels and to add their half sum. In order to determine the phase, one conventionally uses phase locked loops, the various reference values of which are stored by capacitors. The drawbacks of those analog approaches are well known: the component values are critical and require accurate adjustments; the storage of those values is delicate since the latter is sensitive to noise and depends upon the signal content; lastly, those approaches require the use of high value capacitors and an inductance for the phase locked loop oscillator; those components being not integrable, it is necessary to provide on the integrated circuit connection pads for external components.
The conventional digital-type approaches consist in digitally converting the incoming signal with an analog-digital converter (ADC); then, the signal is digitally processed in various ways (correlation, digital filtering, echo cancellation, etc). This approach has the advantage over the digital approach to use integrable components but it exhibits the drawback that the ADC must operate at a high frequency with respect to the input signal frequency which is, conventionally in the TV field, of about a few MHz. Moreover, it will be generally necessary to provide digital multipliers and all those circuits are relatively complex and occupy an important silicon surface.